This project explores the molecular and physiological interactions between two crucial regulators of craniofacial development, bone repair/regeneration, and optimal bone mass accrual and maintenance: Runx2 and Axin2. Runx2 (Cbfa1) is a transcription factor required for osteoblastogenesis and chondrocytes hypertrophy. Runx2-deficiency is postnatally lethal, whereas Runx2-haploinsufficiency causes cleidocranial dysplasia (CCD) and osteopenia. The only known biologic means to at least partially rescue the CCD phenotype in Runx2 mice is by inhibiting Gsk32. Axin2 is the concentration- limiting scaffolding protein that assembles Gsk32, 2-catenin and other components into the 2-catenin destruction complex. Axin2 is a negative feedback regulator of canonical Wnt signaling and slows osteoblast proliferation. Interestingly, Axin2 knockout mice have craniosynostosis (CS) and high trabecular bone mass density. In humans, AXIN2 mutations are linked to familial tooth agenesis. Thus, Axin2-deficiency and Runx2- haploinsufficiency cause opposing craniofacial and trabecular bone phenotypes. We show that Axin2 levels are increased in Runx2-deficient cells. The central hypothesis of this project is that Runx2 actively represses Axin2 to enhance Wnt/ 2-catenin signaling in bone cells. We aim to determine if canonical Wnt signaling is altered in osteoblast progenitors from Runx2-/- mice, define the molecular mechanisms whereby Runx2 regulates Axin2 transcription, and to quantitatively assess the skeletal phenotypes of double mutant Runx2: Axin2-/- mice relative to wild type, Runx2 and Axin2-/- mice.